The present disclosure is related generally to the field of temperature sensors. More particularly, the present disclosure is related to temperature sensors for use on sensing devices.
In some applications, sensing devices are constructed using a metal oxide semiconductor (MOS) resistive sensing material on a thermally isolated microbridge structure (BRIDGE), for example, as the structure for a MOSBRIDGE fire sensor. As will be understood by those of ordinary skill in the art, resistance circuits can be used to measure resistance and that resistance can be indicative of various qualities affecting the circuit.
When a fire starts, the combustion activity produces gases that can be detected by sensing devices. These fire sensing devices are used to measure a change in the gases around the sensing device. This can be beneficial, in some situations, for example where a change in gas can be detected before other signs of fire have occurred in the vicinity, like particulates production in smoke or a significant increase in temperature.
For example, with respect to fire sensing, a MOSBRIDGE based sensing device can be used to interact with gases around the MOSBRIDGE sensor. The sensor can be fabricated from materials that interact with the gases such that some of the gases produced can change the resistance of the material forming the sensor (e.g., the MOS of the MOSBRIDGE).
This change in resistance can be used to identify a changing gas environment around the fire sensor thereby indicating that a fire is changing the gas environment. However, heating of the sensor material can also create a similar resistance condition, and may lead to erroneous fire indication, in some instances.
Typically such devices utilize a material that needs to be heated in order to optimize its sensitivity. In order to heat the material, such devices typically utilize a platinum heating element. One approach to identifying the temperature of the sensing material is to monitor the heating element to determine the temperature of the sensor. However, in some situations, electro-migration from the temperature sensor may disable or destroy the heater. This is particularly true in situations where the devices are miniaturized. For example, a sensor with an area of around 20 microns may have such characteristics although this disclosure is not limited to such sizes.
To remedy this issue, Nickel-Chromium heaters could be used as a substitute for the platinum heaters, but these heaters have low temperature coefficients of resistivity and, therefore, monitoring the heater's resistivity to determine temperature of the sensor may be problematic in some situations.
In designs, the heater can be fabricated having a number of conducting legs extending away from the central portion of the device. Accordingly, these legs provide a portion of the resistance attributable to the heater layer and therefore the heater layer has only a portion of its resistance in the area proximate to the sensing material (e.g., central heated zone of the device) and so changes in the temperature of the heater may not be accurate.
Additionally, this central zone can be small (e.g., 20 microns×20 microns) in some device configurations and, as such, temperature changes can occur quickly thereby making measurement of the heater unreliable in some applications. However, having such a small sensing area can allow for the sensing device to operate under a low power level (e.g., under 10 milliwatts, in some applications) versus other larger devices and therefore, such devices could be useful.